nitrogen doped polycyclic aromatic hydro-carbons

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Well-defined boron/nitrogen doped polycyclic aromatic hydrocarbons are active electrocatalysts for the oxygen reduction reaction Rachel J. Kahan, Wisit Hirunpinyopas, Jessica Cid, Michael J. Ingleson, and Robert A.W. Dryfe Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04027 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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Chemistry of Materials

Well-defined boron/nitrogen doped polycyclic aromatic hydrocarbons are active electrocatalysts for the oxygen reduction reaction Rachel J. Kahan, Wisit Hirunpinyopas, Jessica Cid, Michael J. Ingleson* and Robert A. W. Dryfe* School of Chemistry, University of Manchester, Manchester, M13 9PL, United Kingdom ABSTRACT: Top down synthesized B and B,N-doped carbons (e.g. graphenes) have been previously reported as catalysts for the oxygen reduction reaction (ORR), with activity superior to Pt electrocatalysts also previously reported in some cases. Such doped carbon materials are, however, chemically complex and contain multiple sites which complicates the development of structure activity relationships and thus subsequent catalyst optimisation. Herein, a number of welldefined B and B,N-doped polycyclic aromatic hydrocarbons (PAHs), prepared by a “bottom up” approach, are shown to be active catalysts for the ORR in alkaline solution when deposited on carbon electrodes in contrast to the all carbon based PAH perylene. Six dissimilar B-doped-PAHs have been tested on three working electrodes and the merits of each electrode for assessing ORR catalytic activity determined. A boron doped diamond electrode was found to have the lowest background activity (relative to glassy carbon and HOPG) and thus proved optimal for determining the ORR catalytic activity of the PAHs. Of the six B doped-PAHs studied the two PAHs with the highest LUMO energy were found to be inactive, while the other PAHs with lower LUMO energies were found to be active catalysts for the ORR. Doping of two heteroatoms, doubly B doped and a B,N co-doped PAH containing separate (non-bonded) B and N atoms, was found to lead to the most active ORR catalysts from this set. This suggests that two proximal (separated only by one or two carbons) electrophilic sites improve the ORR activity of doped PAHs. This is the first study, to the best of our knowledge, which uses well defined doped PAHs as models to identify potential ORR electrocatalytic moieties present in doped carbons: this approach thus enables definitive structure activity relationships to be developed in this important area.

The oxygen reduction reaction (ORR) is a crucial process in metal-air batteries and in fuel cells.1 While platinumbased catalysts are currently used for the ORR in fuel cells a combination of high cost, poor long term durability and poisoning (e.g. by CO / MeOH) has hampered mass commercialisation of this technology.1,2 The development of carbon materials that are electrocatalysts for the ORR has attracted significant interest particularly over the last decade as the need for renewable energy alternatives to fossil fuels has increased.3-6 A range of earth abundant p block element-doped carbon materials have been previously reported as ORR catalysts,7,8 including B/N-doped carbons, some of which are reported to display comparable or superior electrocatalytic performance to platinum without the aforementioned drawbacks.9-13 The preparation, characterisation and performance of heteroatom-doped carbon electrocatalysts has been reviewed extensively.8, 14-20 However, the “top down” synthetic approaches produces chemically complex materials containing a range of different functional groups. Furthermore, these approaches do not allow for precise control over dopant content, dopant atom proximity or the chemical nature of the dopant. This has been noted using X-ray photoelectron spectroscopy (XPS), whereby borondoped carbons typically feature BC3, BC2O, BCO2 and B2O3 functionalities;21–25 additional functionalities are observed for boron nitrogen co-doped materials (such as B-N bonds and pyrrolic, pyridinic and graphitic nitrogen functionalities).26–32 Furthermore, it has been demonstrated

that oxo-terminated defect and edge sites can contribute to the ORR activity of graphenes,33-36 and trace metal impurities (in particular iron from commercial graphite and manganese species from the synthesis of graphene oxide by the Hummers method) also can significantly contribute to the ORR catalytic activity of carbon materials.37-39 Indeed, unequivocally discounting metal-impurity catalysis in carbon materials is challenging and often cannot be precluded.40 Combined, these factors have complicated understanding of the precise nature of the key ORR active sites in heteroatom-doped carbon materials. Consequently, the field has had to rely on computational studies to provide insight into potential active sites and the mechanism for the ORR.21, 41-44 However, there still remains significant uncertainty over the identity of the most active sites in doped carbons for catalysing the ORR. An alternative approach to obtain structure-activity relationships in this important area would be to use precisely defined carbon materials containing a small number of dopant chemical sites. In recent years there has been sufficient progress in the preparation of borondoped and boron/nitrogen co-doped polyaromatic hydrocarbons (PAHs) by bottom up synthetic methods to enable this approach to be explored.45-49 As these dopedPAHs are structurally well characterised and have only a small number of heteroatom functionalities, they can be used as well defined models for the functionalities present in larger doped carbon materials. This allows for the cor-

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relation of relative ORR catalytic activity to specific dopant site structure and properties. Herein we report our initial investigations into determining the ORR electrocatalytic activity in alkaline solution of a range of well-defined boron-doped and boron/nitrogen co-doped PAHs. Analysing this series revealed that a minimum LUMO energy is essential, with only the compounds with lower LUMO energies showing electrocatalytic activity for the ORR. Furthermore, out of the four active compounds the two which contain two proximal electrophilic sites show highest catalytic activity.

Figure 1: The well-defined doped PAHs used herein, and the comparison compounds quinone A and perylene (compound B). Mes = mesityl a boron “protecting group”. The compounds 1 – 6 have been previously synthesised and were selected as they are among the largest well-defined B-and B,N-doped PAHs, it should be noted an extremely limited number of these type of complexes have been reported to date. Furthermore, they are relatively chemically robust for di- (and tri-) organoboranes, for example they do not undergo protodeboronation (C-B cleavage) readily with H2O and are stable to silica gel (unlike most triarylboranes).50-54 This is crucial as it maximises the probability of these compounds remaining intact under alkaline ORR reaction conditions, essential for determining structure-activity relationships. However, some of these compounds were unstable to strongly acidic conditions thus this study focuses on alkaline ORR conditions. These compounds also feature comparable boroncontaining functionalities to those postulated to be ORR active in B-doped graphenes (all compounds feature BC3(aryl) units, with the exception of 5 which has a C2BN unit). However, both the number of dopant atoms (1 - 3

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boron atoms, 0 - 1 nitrogen atoms) and the proximity of the dopant atoms vary between the compounds. These compounds therefore represent some of the different configurations available to BC3 and C2BN units within borondoped and boron/nitrogen co-doped carbon materials. The key goal of this initial investigation is to determine the relative activity of these well-defined compounds, thereby enabling correlation of activity with structure. Therefore electrocatalytic conditions that have been used widely in the literature for doped graphenes were employed in this study in order to determine compound activity (see SI). For validation of the methodology, 9,10-phenanthrenequinone (A) was tested alongside compounds 1 – 6. Compound A is known to be an active PAH catalyst for the ORR and its catalytic activity is well understood under the conditions used in this study, making it a suitable reference PAH catalyst.55-58 Furthermore, an all carbon PAH, perylene (compound B) was selected for comparison as it is structurally related to the B,Ndoped PAH 6 thereby enabling further probing of the effect of heteroatom doping on catalysis of the ORR. In addition, carbon electrodes have been shown to have varying degrees of ORR catalytic activity: this includes glassy carbon,59,60 boron doped diamond (BDD),59-62 and highly ordered pyrolytic graphite (HOPG).58, 63-65 Therefore the background activity of these electrodes was analysed prior to compound deposition to understand the contribution to ORR activity from these carbon substrates. Compounds 1 – 6 were synthesised as previously reported and deposited onto a glassy carbon electrode (glassy carbon was selected for initial studies as it is the most commonly used substrate in literature ORR studies) by drop-casting from a toluene solution and drying at ambient temperature. To facilitate evaporation of toluene, the coated electrode was subjected to reduced pressure (10-2 mbar) for several minutes prior to commencing ORR testing. The CVs of compounds 1 – 6 (Figure 2a and 2b) loaded onto glassy carbon show that in air saturated KOH(aq) (0.1 M) ORR activity is actually diminished to varying extents with respect to the bare glassy carbon electrode. Repeating the measurements in N2 saturated KOH(aq) led to featureless voltammograms for these compounds (Figure S1 – S3), illustrating that the observed activity in each case is due to oxygen reduction. It is noteworthy that the ORR activity of the glassy carbon electrode diminishes on compound deposition, presumably due to the PAHs blocking ORR active surface sites. This hypothesis was supported by performing multiple electrocatalysis cycles (Fig. 2c). Cycling results in desorption of the compound and a return to the CV of the original (pre-deposition) glassy carbon. It also should be noted that attempts to use Nafion as a binder to form stable (with respect to compound desorption during cycling) compound deposition on the electrode surface did not lead to reproducible results in our hands (see Supporting Information), therefore in this work all of the voltammagrams are for the first ORR cycle after deposition and drying (as even on cycle 2 and 3 significant compound desorption has occurred – Fig 2c).


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b0.04

a 0.04

0.00

0.00

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-0.04

c 0.03

0.00

GC 1 2 3 4

-0.12

-2

J / mA cm

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cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 cycle 10 cycle 20 cycle 40 cycle 75

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-0.16 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Potential / V vs Hg/HgO

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0.0

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0


-1

Figure 2. (a) CVs of 1 – 4 and (b) 5 – 6 on a glassy carbon electrode in air saturated 0.1 M KOH(aq) at a scan rate of 50 mVs . The CVs of the bare GC electrode are shown by the grey dashed lines. (c) CVs of 4 on a GC electrode in air-saturated 0.1 M KOH(aq) at -1 a scan rate of 50 mVs illustrating desorption of the compound from the electrode. GC = glassy carbon.

With initial work using these PAHs deposited on glassy carbon proving inconclusive (with respect to any PAH ORR activity) due to the relatively high background activity of the glassy carbon substrate other substrates with lower intrinsic ORR activity were investigated. Other carbon substrates that have been studied extensively for the ORR include BDD58-62 and HOPG,58, 63-65 and both of these have significantly reduced ORR activity in comparison to glassy carbon. Boron doped diamond is reported to be completely inactive for the ORR (any ORR activity in BDD is attributed to traces of sp2 carbon present in the substrate).62,66 Therefore BDD was studied to determine its suitability as an electrode for the assessment of the six doped PAHs as catalysts for the ORR. The CVs of two

Figure 3. Top, CVs of A on a glassy carbon electrode in N2 -1 and O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs . Bottom, CVs of A on a BDD electrode in N2 and O2 saturated -1 0.1 M KOH(aq) at a scan rate of 50 mVs . The CVs of bare glassy carbon and bare BDD are shown by the dashed lines.

commercial BDD electrodes show minimal ORR activity and importantly, show no reductive features around -0.4 V in air or O2 saturated KOH (Figure S7). Notably, the ORR activity of PAH A deposited on BDD is similar (although with a slightly lower onset potential) to the intrinsic ORR activity of glassy carbon consistent with surface quinone groups in glassy carbon being involved in the ORR (Figure 3). Therefore BDD electrodes are more useful for this study as they provide a background with lower intrinsic activity than the glassy carbon electrode, thereby facilitating the determination of ORR activity of compounds 1 – 6. The six compounds again were deposited on the BDD electrodes by drop-casting a toluene solution of the compound followed by evaporation of toluene at low pressure. The CVs after depositing compounds 1 – 6 onto BDD all show no features in N2 saturated KOH(aq) (see Figure S11), but in O2 saturated KOH solutions significant differences were observed (Figure 4a-b). Compounds 1, 3, 4 and 6 loaded onto BDD gave CVs with increased ORR activity relative to the bare BDD electrode. In contrast, BDD deposited with 2 and 5 showed less, or effectively identical, ORR activity to that of the bare BDD electrode. This indicates for the first time that well defined B-doped and B,Nco-doped PAHs are active catalysts for the ORR. Of the active doped PAHs, 3 and 4 showed the lowest increase in activity with 3 having a higher onset potential than the other active compounds. By comparison, 1 and 4 have lower onset potentials than 3, while 1 has a higher current density than both 3 and 4 (Table S3). The CV of compound 6 showed the highest ORR activity of all six compounds investigated in this study with a low onset potential and a comparable current density to 1 at -1.0 V vs Hg/HgO.67 Perylene (B) can be viewed as a structurally related all carbon PAH analogue of compound 6. However, with perylene deposited onto the BDD substrate the ORR activity (figure 4c) is poor and not significantly increased compared to the background activity of the BDD substrate. This clearly highlights the importance of heteroatom doping in these systems for catalysing the ORR. It should be noted that whilst the compounds selected for this study are bench stable (due to steric protection of the organoborane by the mesityl group or by structural



-0.05

-0.05

-0.10

BDD 1 2 3 4

-0.15

-0.20

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

J / mA cm

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-0.09 -0.11

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HOPG 0.5 µL 6

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0.0

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HOPG 2.0 µL 6

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HOPG 0.5 µL A

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-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Potential / V vs Hg/HgO -1

Figure 4. (a) CVs of 1 – 4 and (b) 5 – 6 on a BDD electrode in O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs (for separate plots of the CVs of 1-6 also containing the background CV and CV under N2 in each case see Figure S12) . (c) CVs of B on a BDD -1 electrode in N2 and O2 saturated 0.1 M KOH(aq) at a scan rate of 50 mVs . The CVs of the bare BDD electrode are shown by the -1 grey dashed lines in a-c. (d-e) CVs of 6 and (f) CV of A on HOPG in air saturated 0.1 M KOH(aq) at a scan rate of 50 mVs . The CVs of the bare HOPG electrode are shown by the grey dashed lines.

constraint around the boron centre)50-52 these species may still undergo decomposition (e.g. protodeboronation) under more forcing conditions such as in the presence of aqueous bases.68-70 To provide support that the ORR activity observed is due to the boron doped PAHs and not any decomposition products, the adsorbed compounds were extracted from the electrode surface (using toluene) after electrocatalytic testing and analysed by mass spectrometry (see SI). This confirmed the presence of the molecular ion (e.g. for compounds 1 and 4) or (M+OH)− species (e.g. for compounds 5 and 6). Ions correlating to anticipated decomposition products were not observed for any of the compounds, suggesting that under these ORR conditions these compounds do not undergo significant B-C cleavage or other decomposition. Each of the doped PAHs are synthesised via at least one metal mediated step prior to purification by column chromatography, however, the absence of any significant trace metal impurities was confirmed by ICP-MS, (including Fe, Pt, Mn, Co and Ni all of which are documented to catalyse the ORR).37-39 Furthermore, the purity level of these PAHs is > 99% (by multinuclear NMR spectroscopy) further disfavouring impurity catalysis. Combined these observations support the conclusion that the observed ORR activity on BDD is due to the B-doped PAHs. Repeated attempts to determine the value of n for the ORR process were frustrated by a lack of dependence on scan rate (CV), however for the best performing model catalyst, 6, RRDE voltammetry with a bespoke apparatus (BDD/catalyst disk and Pt ring) yielded stable disk and ring currents. The number of electrons transferred in

Figure 5: A RRDE voltammogram obtained at Pt/Pt ringBDD disk assembly in 0.1 M KOH at 1600 rpm at room temperature. The experiment was carried out in saturated oxygen conditions. The BDD disk potential was scanned from 0 V to 1.2 V at 5 mV/s and the ring potential was 0.1 V.

the catalytic process using 6 was 2.65, indicating that mixed pathways operate (potentially dependent on compound surface coverage), but a portion of the oxygen is reduced to water under these conditions (Figure 5). PAH 6 has the highest observed electrocatalytic activity on BDD, therefore the ORR performance of this compound was also studied on HOPG. Bare HOPG displays a small ORR feature around -0.4 V vs Hg/HgO with an onset potential in the region of -0.3 V, which is consistent


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with other reports of ORR catalysed by HOPG.61 The adsorption of 6 onto HOPG results in a negative shift in the reduction potential (to a value consistent with that obtained for 6 from the BDD electrode). Thus 6 is less active than surface groups on HOPG, but the activity of 6 can clearly be determined, with it suppressing the intrinsic activity of surface HOPG sites. This demonstrates that ORR active PAHs can also be investigated on this substrate (Fig 4d and e), although BDD remains the preferred substrate due to its lower and more negative background activity. For comparison compound A was also adsorbed onto HOPG and the onset and peak potentials of this compound (-0.284 V and -0.421 V, respectively) further support evidence in prior literature that the ORR active groups on HOPG are quinone-based.57 From the above studies it also can be concluded that BDD is the preferred substrate for determining the electrocatalytic activity of PAHs towards ORR. While glassy carbon is ubiquitous in the literature and offers a stable, reproducible background over a series of measurements, the relatively high intrinsic ORR activity of this substrate can complicate unequivocal determination of the ORR activity for any compounds with a higher onset potential and/or lower current density than catalytically active surface groups. HOPG offers a background with lower intrinsic activity than glassy carbon substrates; however the background activity observed is dependent on the number and type of defect and edge sites.61, 63 Furthermore, the variation between measurements on HOPG (due to removal of the surface layer(s) in between each use) can complicate accurate comparison of electrocatalytic activity between compounds deposited on HOPG (e.g. see background CVs in Figure 4d, e and f, and Table S4). Therefore BDD offers the best (least ORR active) background of the three carbon substrates and provides reproducible background values over a series of measurements. Table 1: Select properties of compounds 1 – 6 and A. HOMO (eV)a

LUMO (eV)a

Epeak

Dipole

1st red (V)b

Moment (D)a

Onset Potential (V)c

1

-6.47

-2.28

-1.56d

0

-0.35

2

-6.91

-1.81

-2.10e

1.00

-

3

-6.91

-2.35

-1.56e

0.39

-0.33

4

-6.80

-2.37

-1.63f

6.37

-0.31

5

-7.34

-1.30

-

0.04

-

No.

e

6

-7.52

-2.08

-1.80

5.13

-0.32

A

-8.07

-2.36

-1.25

7.68

-0.27

a = calculated frontier orbital energies. b voltage at peak current for the first reduction process, potentials are given rela+ tive to the Fc/Fc redox couple. C = onset potential on BDD for ORR active compounds. The onset potential in the absence of substrate for the intrinsic activity of the BDD substrate was -0.63 V. These are relative Hg/HgO. c = from ref 50, d = from ref 51, e = from ref. 54. For reference the E1/2 of + 72 perylene (B) is reported as -2.12 V relative to Fc/Fc .

The relative ORR catalytic activity of the six doped PAHs analysed in this study can be grouped into three subsets, compounds 1 and 6 = most active, 3 and 4 = some activity, and 2 and 5 = not-active for the ORR. It should be noted that while a number of these B and B,N-doped PAHs are active catalysts for the ORR they are less active than A and previously reported B/N doped graphenes. Nevertheless, these compounds do reveal important structure activity relationships.8 Key properties of these six PAHs are summarised in Table 1 with DFT studies performed at the M06-2X/6311-G(d,p) level, with this functional chosen as it was parameterised to give accurate data for p block element containing compounds.71 Charges are from Natural Bond Orbital (NBO) calculations. From inspection of the frontier orbital energies (from calculations and cyclic voltammetry) it can be seen that compounds 2 and 5 have the highest LUMO energies. Therefore the lack of ORR activity can be attributed to the high energy LUMOs which will result in low Lewis acidity of these two PAHs (i.e. the binding energy of O2 and O2 derived species can be expected to be low), and a more negative reduction potential. This is consistent with previous computational studies on heteroatom doped carbons which suggest that oxygen reduction proceeds via binding of oxygen to the heteroatom (for heteroatoms such as boron)44 or a neighbouring electrophilic carbon site (for heteroatoms such as nitrogen).41, 73,74 Therefore in doped-PAHs it is proposed that for oxygen binding/reduction to occur the PAH must have a sufficiently low in energy LUMO. This is also consistent with the ORR activity of quinone A which has the lowest LUMO energy (by cyclic voltammetry) and the lowest onset potential for the ORR. However, comparing the four active B-doped PAHs, although compound 6 has the highest ORR activity this compound does not have the lowest LUMO energy of the four active B-doped PAHs. It is therefore clear that once the LUMO energy is sufficiently low other factors, presumably the presence of specific chemical functionality, also significantly contribute to the ORR activity. The importance of specific functional groups/dopant atom proximity is exemplified by comparison of the ORR activity of 1 and 3. These two PAHs are similar in terms of boron content, PAH size (excluding the OMes groups on 1 which do not contribute to the frontier molecular orbitals, Fig. 6, top left) and LUMO energy, but have different ORR activities. Compounds 1 and 3 have similar charge distribution (positively charged boron centres and negative, ca. -0.4 e, carbon centres); with a LUMO that has significant boron character in both cases. The higher activity of 1 relative to 3 (and 4) therefore is proposed to be due to the proximity of the two Lewis acidic positively charged boron centres which can feasibly both bind to a single O2 molecule (or an O2 derived species) during the ORR. Indeed an O2 adduct of a related 9,10-dibora- anthrene compound has been recently reported with one O2 molecule binding to both proximal boron sites (inset, Fig. 6).75 Calculations on boron-doped graphene



Figure 6: LUMO (at isosurface value =0.04) and select NBO charges for compounds 1, 3 and 6. Inset, the product from reaction of a NHC-stabilized 9,10-dibora-anthrene with O2.

nanoribbons also found that the proximity of multiple boron sites positively affects the ORR activity, with boron atoms in a 1,4-configuration in a six membered ring (such as in 1) showing increased ORR activity due to higher affinity for oxygen adsorption, again presumably due to O2 (or O2 derived species) binding to both boron atoms facilitating the ORR.44 The relatively high activity of compound 6 is also consistent with a heteroatom co-doping effect, with one boron centre and a positively charged carbon (either ortho or para to N) being effective proximal Lewis acidic sites. Inspection of the LUMO for 6 revealed it has significant character on boron and the carbon para to nitrogen, suggesting that these two are potential sites for bidentate O2 binding. Furthermore, the incorporation of N into 6 leads to more charge polarisation with the magnitude of positive charge on boron now greater than that found in 1, 3 and in 5 where boron is directly bound to nitrogen (see Supporting Information). Boron / nitrogen co-doped carbon materials are documented to be excellent catalysts for the ORR and it has been suggested that separated (not directly bonded) boron and nitrogen dopants have high ORR activity partly due to the redistribution of electron density leading to more charge polarised systems.12 Previous calculations have found the strength of the co-doping synergistic effect decreases as the distance between dopant atoms increases, thus proximal (e.g. separated by only 1 or 2 carbons) B and N are potentially required for concerted binding of O2 (or O2 derived species) and thus effective ORR catalysis.13 The higher electrocatalytic activity of 1 and 6 in this series supports the hypothesis that two heteroatom dopants helps to promote high ORR activity by generating two proximal electrophilic sites. In summary, these initial studies have shown for the first time that well-defined boron and boron nitrogen co-

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doped PAHs are active electrocatalysts for the ORR in alkaline solutions provided the LUMO energy is sufficiently low. Furthermore, in this series of compounds the presence of two dopant atoms in relatively close proximity leads to enhanced ORR activity for both doubly B doped (in compound 1) and B,N co-doped (in compound 6) PAHs. Notably, a related all carbon PAH, perylene, that is structurally related to 6 displays essentially no electrocatalytic activity under these conditions. Multiply heteroatom doped PAHs therefore have significant potential as model compounds for exploring the effects of specific functionalities on the electroactivity of doped-carbon materials. It is acknowledged that for more in-depth understanding of the nature of active functional groups, more compounds (with different functionalities/arrangement of dopant atoms / LUMO energies / LUMO distributions to those studied herein) need to be studied, but this work suggests that larger co-dopedPAHs with lower LUMO energies are attractive targets. Furthermore, the study presented herein illustrates that an effective screening method for assessing doped PAHs for ORR activity requires understanding of the relative activity of the electrode substrate and the compound of interest. This has led to B and B/N doped PAHs being confirmed as active catalysts for the ORR for the first time.

ASSOCIATED CONTENT Supporting Information. The Supporting Information listed below is available free of charge on the ACS Publications website at DOI: Materials and methods, electrochemical measurements, characterization of boron doped diamond electrodes and computational details (PDF).

AUTHOR INFORMATION Corresponding Author * Michael J. Ingleson [email protected] * Robert. A. W. Dryfe [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The EPSRC The Leverhulme Trust The European Research Council.

ACKNOWLEDGMENT We are grateful to the EPSRC (EP/K03099X/1), the ERC (Grant No. 305868) and the Leverhulme Trust (RPG-2014340). Additional research data supporting this work are available as supplementary information accompanying this publication.

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